An exciting development in 2006 is the design of a specialised switch that will detect if the primary external aircraft antenna has been damaged during a crash landing. A device called a SASD through which the signal from the 406MHz ELT passes achieves this. The SASD is passive under normal circumstances and if the external or primary antenna is operational, the SASD is transparent. However, if the SASD senses that the primary antenna is damaged, it will switch transmissions to an auxiliary or secondary antenna mounted either on the SASD or adjacent to it. At that point the control circuit will ensure all further transmissions both 406MHz, 121.5 and 243MHz are transmitted via the auxiliary antenna.